Fail-safe ignition circuit for detonating explosives in well bores



May 8 1962 w. R. GIESKE ETAL 3 FAIL-SAFE IGNITION CIRCUIT FOR DETONATING EXPLOSIVES IN WELL BORES Filed March 25, 1958 2 Sheets-Sheet 1 INVENTORS. W/LL /4M 1?. G/E'SKE May 8, 1962 w. R. GIESKE ETAL FAIL-SAFE IGNITION CIRCUIT FOR DETONATING EXPLOSIVES IN WELL BORES 2 Sheets-Sheet 2 Fileq March 25, 1958 FREE/ 00V? lA/O/CATOAZ CYECU/T POWER SUPPLY 04 AAA? INVENTORS. W/LL/AM 2. 5/55/(5 BY 77/500025 L. BERRY 7'7 A/EK MPENSATED TIME DELAY FAIL-SAFE lGNlTI GN CiRQUlT FOR DETGNAT- lNG EXPLGSIVES IN WELL BORES William R. Gieske, Fullerton, and Theodore L. Berry,

Santa Fe Springs, Calif., assignors to The Ford Alexander Corporation, Whittier, Calif., a corporation of California Filed Mar. 25, 1958, Ser. No. 723,875

laims. (Cl. 102-26) 1 This invention relates to oil well tools, and more particularly to electrical apparatus for selectively detonating an explosive or supplying electric power to another circuit connected to the same two-conductor transmission line without detonating the explosive.

The usually prominent character of the safety problem involved in performing some other electrical operation first and subsequently detonating an explosive by the ap-. plication of an electric voltage to the same transmission line is rather obvious. It is this problem which the invention overcomes. Detonation of an explosive in an oil well by no means creates a minor hazard and unquestionably endangers personnel working nearby if the detonation is not properly controlled.

Explosives are used in oil wells for many reasons. Although the apparatus of the invention may be employed for various purposes and it is not to be limited to any particular one, it is especially useful in the detonation of an explosive used in a process known as a back-ofi process performed to unscrew drill pipe or tubing to recover itfrom an oil well.

In the back-ofi process, an oil well tool with'nn explosive, called a string shot is lowered into a well on a cable and positioned adjacent a tool joint. The cable normally has an outer conductive armor and an inner conductor insulated from the outer conductive armor. In the back-off process, then, torque is applied to the pipe at the top of the well in a direction to unscrew it, and a voltage is applied between the conductors of the cable to detonate the string shot to jar the tool joint so that the pipe may be unscrewed by the torque applied to the pipe at the top of the well.

It is impractical to use more than a two-conductor cable to act as a two-conductor transmission line and to lower a tool because of mechanical limitations of size and tensile strength and electrical limitations of resistance. However, it is also desirable to perform more than one electrical function on the same two wire line.

For example, if a pipe becomes stuck in a well, it is desirable to recover a maximum length of the pipe above the stuck point so that washover pipe may be lowered over the upper end of a stuck section in close proximity to the stuck point to free the stuck pipe. For this reason, tools called free point indicators are lowered in pipe to determine the depth of a stuck point accurately before the pipe is parted at a tool joint immediately above the stuck point by the back-off process.

If a stuck point is 10,000 or 12,000 feet in a well, it may take several hours to make one trip down and back with the free point indicator. This time is effectively doubled when it becomes necessary to lower the string shot for backing oil free pipe from the stuck pipe. Thus a substantial advantage can be derived in connecting both the free point indicator and a string shot to a wire line, lowering them both into a well, operating the free point indicator first to determine the position of the stuck point of pipe, and subsequently detonating the string shot at the tool joint immediately above the stuck point.

Another situation also often occurs in which it becomes desirable to perform two electrical functions sequentially on a single two-wire transmission line. Specifically, drill collars, heavier and therefore larger than drill pipe, are

tates a tnt 3 3,033,114 Patented May 8, 1962 ice very often the cause of a pipe becoming stuck. For this reason, it is often desirable to back-off a drill collar. Due to the size and weight of a drill collar, it is necessary to use a string shot substantially larger than that which is employed to back-otf drill pipe. Still further, the backotf process fails most often when it is employed in backing off drill collars. Thus, if a drill collar is stuck in a well at a depth of 10,000 or 12,000 feet and a string shot fails to jar a free drill collar loose, it often becomes necessary to make a second time-consuming trip into the well with a second string shot to back-off drill pipe immediately above the top drill collar of the string. It is then obvious that it could be very advantageous to detonate two string shots on the same two-conductor cable, the first being employed to back-off a drill collar, and the second being employed to back-off drill pipe if the first string shot was ineffective to jar the drill collar loose.

Although it may be desirable to perform an electrical operation on the conventional two-conductor cable in addition to that of detonating an explosive, as stated previously, detonation itself is hazardous at the very least. Thus when the chance for accidental detonation is increased by the use of additional electrical apparatus on the same two-conductor cable, the hazard is further increased. Moreover, when two string shots are connected to the same transmission line to be detonated sequentially, the

hazard is obviously extremely great.

For example, if a string shot is connected across a two-conductor transmission line and the line is shorted out at the surface of the well, accidental detonation may take place simply by induced currents. Galvanic action between circulation fluids in a well and the conductive armor of the cable can also create surprisingly dangerous currents. Accidental ignition also can take place in other ways. Such can happen when a switch such as a relay is employed in a tool to be lowered into a well to connect a string shot with the transmission line to be fired when a pair of its normally open contacts close. Thus if salt water should seep into the tool, the normally open contacts could be electrically connected by the salt water electrolyte even though the relay were not energized.

The present invention overcomes the above-described. and other disadvantages of the prior art by providing a tool to be lowered into a well including a two-conductor transmission line; a load impedance responsive to at least a first type signal applied to the transmission line connected across the line; an explosive charge responsive to at least a second type sign-a1 applied to the transmission line; and switch means normally connecting a shorting circuit across the explosive charge, the switch means being actuab-le only in response to the application of the second type signal to the transmission line both to open the shorting circuit and to connect the explosive charge across the transmission line.

Thus by maintaining the explosive charge normally shorted deep in a well, the possibility of accidental detonation by induced currents is minimized because the physical length of the short circuit provided by the switch means is relatively small in comparison to the total length of the cable when it is lowered into a well. Accidental detonation by currents generated by galvanic action is also minimized because of the relatively short length of the shorted circuit of the explosive charge. Still further, salt water cannot cause accidental detonation because the explosive charge is always shorted until the switch means is energized.

energize the time delay relay in a tool below, and output voltage of the source may be varied until the ammeter indicates a current adequate to fire the string shot properly. The relay may have a time delay of a length sutficient to permit this adjustment before a firing voltage is actually impressed on the explosive by the action of the time delay relay armature falling into place and closing an electrical circuit from the variable voltage source to the explosive.

According to an aspect of the invention, the relay may be a temperature-compensated thermal time delay relay. Thus, widely varying temperatures encountered in lowering a tool in an oil well will not afiect the efiicient operation of the relay.

It is also a feature of the invention that a fail-safe system is provided for selectively supplying electrical power to a load impedance, whether it is the free point indicator or a second string shot or other device, and to a first string shot. This system not only includes the switch means with the shorting circuit, but also one or more additional means to prevent accidental explosive detonation. One of these is a series connected rectifier to prevent detonation except on the application of a voltage of a specific polarity. Another of these is a power supply to be utilized at the top of a well to impress voltages of different amplitudes on a transmission line extending into the well. For example, the free point indicator may be operated at a substantially lower voltage and current than the voltage and current required to detonate the first string s..ot.

The above-described and other advantages of the invention may be better understood when considered in connection with the following description.

In the accompanying drawings, which are to be regarded as merely illustrative:

FIGS. 1A, 1B, 1C and ll) are side elevational views of a combination tool employed with the circuit of the invention;

FIGS. 2A and 2B are side elevational views of another combination tool employed with the circuit of the invention; and

FIG. 3 is a schematic diagram of the circuit of the invention.

In FIGS. 1A and 18, a portion of a tool is shown which is to be lowered into a well directly above a tool indicated in FIGS. 1B, 1C and 1D. In FIG. 1A, a wire line 1% is indicated connected to a socket ill which is threaded to a housing 1?. which may be made fluid tight to contain most of the circuit of the invention. A string shot 13 is connected from housing 12 to be detonated by the circuit of the invention, string shot 13 being a conventional means of constructing an explosive charge for performing the back-off process. A continuation of wire line it is shown below string shot 13 and above a set of weights 1 in PEG. IE to urge a free point indicator indicated at 15 in FIG. 1D down into a well.

Wei hts 14 are necessary in order to urge free point indicator 15 down into a well because belly springs in and 17 are located thereon to frictionally engage a well bore. The operation of free point indicator 15 generally is well known in the art. An inductor coil is located internally of it which has a magnetic circuit that moves in response to movement of the belly springs 16 and 17 with respect to each other to indicate whether pipe in a well is free at that point when it is pulled upwardly or twisted at the top of the well. Thus, when free point indicator 15 is located below a stuck point of pipe, no movement will be registered and the position of the stuck point may be accurately determined.

After the stuck point of pipe has been accurately determined, wire line 1% may be lowered so that a collar locator, which is simply an inductor coil 23 inside housing 12., may be passed downwardly through the well to determine the existence of a collar or tool joint immediately above the stuck point of pipe. The collar is easily detected in accordance with well known methods.

Specifically, as inductor 23 is lowered in pipe, discontinuities are created in the magnetic flux path thereof by finite higher permeability air gaps that always exist between a collar and a section of pipe at their threaded connection. After the collar above the stuck point has been located, wire line it is again moved to position string shot 13 adjacent the collar and string shot 13 is detonated by the application of a voltage pulse between the center conductor of wire line It), not shown, and its insulated external conductive armor.

in FIGS. 2A and 2B, upper and lower string shots i3 and 13A are shown which may be similar. However, as explained previously, lower string shot 13A may be larger in view or" the fact that often drill collars require a larger string shot to loosen them in the back-off process.

the combination tool in FIGS. 2A and 23, similar apparatus is employed on the same wire line 1'8. As shown in FIG. 2A, the same socket 111 with housing l2 may be employed immediately above string shot 13 on wire line litl. Thus, the structure shown in FIG. 2B is connected directly below that shown in FIG. 2A, the structure shown in R6. 2B also being connected to wire line It This structure includes a socket 11A which may be identical with the socket ill, a housing 12A which may be identical with the housing 12, and a string shot 13A.

Thus, when a stuck point of pipe is determined to be at a drill collar on the lower end of a drill string, the apparatus shown in FIGS. 2A and 28 may be lowered into a wall and string shot 13A positioned adjacent the tool joint of the drill collar immediately above the point where the pipe is stuck. String shot 13A is then detonated while charge 13 remains intact. If the drill collar immediately above the stuck point is not loosened and string shot 13 is smaller than string shot 13A but sufiicient to loosen a tool joint between sections of drill pipe, wire line it? may be raised to position string shot 13 adjacent the first drill pipe tool joint above the drill collars. String shot 13 may then be fired to jar the tool joint loose in the back-off process where unscrewing torque is applied to the drill pipe at the top of the well at the time of detonation.

As explained previously, the hazard of detonating a string shot 13 in an oil well is rather obvious. Still further, it is obvious that the hazard is substantially increased by the detonation of string shot 13A independent of string shot 13 as shown in FlGS. 2A and 2B. The circuit of the invention shown in FIG. 3 is employed to detonate string shot 13 whether it is incorporated in the embodiment of the invention shown in FIGS. 1A, 1B, 1C and 1D or in the embodiment of the invention shown in FIGS. 2A and 2B.

The invention shown in FIG. 3 includes a direct-current power supply 18 having an output voltage amplitude adjustment at a knob 19 to provide a variable amplitude direct-current output voltage with a polarity indicated at the output of power supply 18. Similarly, a collar locator circuit 20, a free point indicator circuit 21, and a free point indicator reset circuit 22 are indicated as blocks in PEG. 3. Collar locator circuit may be provided with a separate power supply and meters to indicate a change in inductance of the total circuit of transmission line It so indicated in FIG. 3 when the combination tools shown in FIGS. 1 and 2 are lowered into a well where a change in inductance of inductor 23 is provided because of the higher permeability path of tool joints in proximity to which inductor 23 is passed. Collar locator circuit 29 need not incorporate a power supply unless an amplifier is used; however, during the operation of this circuit a current will be generated by inductor 23 which creates at least somewhat of a hazard in the accidental detonation of string shot 13. Inductor 23 may be incorporated in housing 12 in both FIGS. 1A and 5A and also as indicated in FIG. 3 in a dotted box. The collar locator circuit 25 may be of the type shown in United States Patent No. 2,717,039 issued September 6,

1955, to W. R. Gieske for Detector Device for Exploring Ferromagnetic Structures in Well Bores.

A free point indicator circuit 21 may also incorporate a separate power supply and means to indicate a change in a load impedance 24 connected across transmission line at the bottom thereof as shown in FIG. 3. Freepoint indicator circuit 21 may be of the type disclosed in copending application Serial No. 656,481 filed April 29, 1957, for a Free-Point Indicator for Determining the Point at Which Stuck Pipe Is Free in a Well by Theodore L. Berry, or of the type disclosed in US. Patents Nos. 2,698,920 and 2,817,808 issued to William R. Gieske. However, it is to be understood that the load impedance 24 may either be an inductor located in free point indicator or string shot 13A. Thus when pipe is pulled upwardly at the top of a well and belly springs 16 and 17 move apart to increase or decrease the permeability of the magnetic circuit of the inductor in free point indicator 15 if it is load impedance 24, a change in the total inductance throughout the circuit of transmission line It may be registered on a suitable indicator in free point indicator circuit 21.

Afree point indicator reset circuit may be employed to reset the free point indicator 15. Free-point indicator reset circuit 22 may be of the type disclosed in said copending Berry application. This is merely an alternative input provided to transmission line 10 and forms no part of the invention. For this reason, the indicator reset circuit connection in transmission line It? is not shown in FIG. 3. A selection is made as to which circuit 21 or 22 is connected to transmission line 1% at a double pole, double throw switch 25. A second selection may also be made as to whether one of circuits 21 and 22 will be connected to transmission line 10 or whether power supply 18 or collar locator circuit will be connected to transmission line 10. This latter selection is made by means of a double pole, triple throw switch 26. Switch 26 selectively connects transmission line 10 to the output leads of power supply 18, or to the output leads of collar locator circuit 22, or to the poles of double-pole, double-throw switch 25.

Transmission line 10 is provided with two conductors 27 and 28. The general series circuit of conductors 27 and 28 is as follows starting from the connection of conductor 27 to switch 26. First occurs collar locator inductor 23. Next serially connected is a resistor 29. A rectifier 30, a relay winding 31 and an adjustable resistor 32 are connected in a series circuit 33, which, in turn, is connected in parallel with resistor 29. Rectifier is poled to be conductive only when a voltage is applied to conductor 27 which is negative with respect to the voltage applied to conductor 28. However, it is desirable to operate on transmission line 10 when a voltage applied to conductor 27 is positive with respect to conductor 28. For this reason, bypass resistor 29 is con nected in parallel with series circuit 33. Further tracing the circuit of transmission line 10, conductor 27 continues downwardly and at its lower end is connected to load impedance 24. Going upwardly from load impedance 24 on conductor 28, conductor 28 is preferably the grounded side or conductive armor of wire line or cable 10 and has no serial connection with any other circuit element. A resistor 34 is connected directly in parallel with load impedance 24 in housing 12. In the event that load impedance 24 takes the form of string shot 13A, occasionally conductors 27 and 28 will blow apart. Although relay winding may be connected directly across transmission line 10 between conductors 27 and 28, in the particular instance shown in FIG. 3 it is not. Thus, a separate current path must be provided in parallel with load impedance 24 when it is string shot 13A and string shot 13A is fired prior to string shot 13 in the event that conductors 27 and 28 at the bottom of transmission line 10 are blown apart opening the circuit to relay winding 31.

As shown in FIG. 3, string shot 13 is incorporated in both the embodiments of the invention. String shot 13 is connected from a contact 35 to a pole 36 from which an arm 37 is pivoted to engage a normally open contact 38 when relay winding 31 is energized, arm 37 being operated by energization of relay winding 31. Contact 35 is normally in engagement with arm 37 and therefore in electrical contact with pole 36. Pole 36 is electrically connected to contact 38 when relay winding 31 is energized and arm 37 is urged into engagement with contact 38. As can be seen, contact 35 is connected both to one side of string shot 13 or the detonating cap therefor and to conductor 28 of transmission line 10. Pole 36 is connected to the other side of string shot 13. Nor mally open contact 38 is connected to one side of relay winding 31.

As stated previously, power supply 13 supplies a direct-current voltage. This is also true of indicator reset circuit 22. Alternating voltages are impressed on trans mission line 10 by collar locator and free point indicator circuits 2i) and 21. It is a feature of the invention that all the voltages impressed on transmission line it? by circuits 2t 21 and 22 are generally insufficient to detonate string shot 13 when load impedance 24 is the inductor coil of free point indicator 15. When load impedance 24 is string shot 13A, only power supply 18 need be provided. Thus there is a voltage amplitude safety factor involved in the relative voltage amplitude supplied by power supply 18 and circuits 2t), 21 and 22.

Probably by now, it is already obvious that energization of relay winding 31 causes string shot 13 to be detonated. It is another outstanding feature of the invention that, combined with amplitude safety, a polarity safety is provided in that free point indicator reset circuit 22, when switches 25 and 26 are closed to connect it to transmission line it will not detonate string shot 13 eventhough voltage applied were of a sufiicient magnitude because rectifier 30 is connected serially with relay 31 and, with the polarities shown at the output of free point indicator reset circuit 22, it is impossible to energize relay winding 31 because rectifier 30 is poled in the wnong direction for energizatio-n. It is to be noted that this is true regardless of whether load impedance 24 is the inductor of free point indicator 15 or string shot 13A. That is, free point indicator reset circuit 22 may be employed to detonate string shot 13A when load impedance 24 is, in fact, that string shot.

Relay winding 31 is a part of a relay generally indicated at 39, pole 36, arm 37, and contacts 35 and 38 being the remainder of relay 39. Relay 39 may be a conventional relay a-ctuable immediately upon the application of a voltage to winding 31. However, an added safety advantage is provided in that relay 39 is a temperature compensated thermal time delay relay. The wide variation in temperature as a tool is lowered into a well is well known in the art. Hence, temperature compensation can be very important. It is to be recognized still further that if relay 39 is a thermal time delay relay, it has a natural period variation with temperature which may be easily compensated for because of the manner in which its time delay is produced. Of course, time delay relays are old and well known in the art. Thus, time delay relay 39 after energization will only cause arm 37 to open contact 35 and pole 36 and close contact 38 and pole 36 a predetermined time after it is energized as is well known.

It is a feature of the invention that all the above safety features may be used singly and especially all in the same combination, all in combination providing an unusually safe circuit. However, it is an outstanding feature of the invention that contact 35 with pole 36 provides shorting contacts for string shot 13 as long as relay 39 is not energizecl.

In the operation of the invention as shown in FIG. 3 with the tool indicated in FIGS. 1A, 1B, 1C and 1D, the tool is lowered into a well and perhaps free point indicator reset circuit 22 is connected at transmission line 10 via' switches 25 and 26 and free point indicator reset circuit 22 operated to apply a direct-current voltage to transmission line 169 of a polarity opposite the direciton rectificr 30 is poled. In this case, relay winding 31 will not be energized but free point indicator may be reset. Free point indicator reset circuit 22, however, may impress a voltage on transmission line it of an amplitude both in suficient to energize relay winding 31 and of a wrong polarity with respect to the direction in which rectifier St? is poled.

After free point indicator 15 has been reset, force may be applied to pipe at the top of the well by torquing it in a screwing direction or by pulling it upwardly to cause one of the belly springs 16 and 17 to move relative to the other. In this case, load impedance 24, being an inductor in free point indicator 15, will have a difierent impedance depending on change in location of magnetic structure surrounding it. Free point indicator circuit 21 will then be connected to transmission line ltl via switches 25 and 2:6 and the change in inductance of load impedance 24 may be metered.

If the above operation is repeated at successive depths in drill pipe or tubing, a point of struck pipe may be determined. In this case, collar locator circuit is connected to transmission line 16 via switch 26 and the tool shown in FiGS. 1A and 1B is raised in the well to determine the position of the collar or tool joint immediately above the stuck point. After this has been determined, wire line 10 is lowered or raised to position string shot 13 adjacent the tool joint. Thus power supply 18 is connected to transmission line it via switch 21'. If desired, Since relay 3/9 is a time delay relay, knob 19 may be turned to adjust the output voltage of power supply 19 to such an extent that an ammeter A connected serially with power supply 18 may register a current sufficient to properly detonate string shot 13. Ample time may be provided to do this before arm 37 is actually moved from the normal position shown to a position in which contact 33 is connected to pole 36 to fire string shot 13. This is a feature of the invention incorporating the time delay of relay 39 Finally, relay winding 31 will be energized by a voltage of an appropriate polarity and magnitude provided by power supply 18 and arm 37 will engage contact 38 and pole as will be electrically connected therewith. It is to be noted that contact 3% is effectively connected to conductor 27 of transmission line 10 whereas contact 35 is effectively connected to conductor 28 of transmission line It Hence, upon encrgization of relay winding 31 and movement of arm. 37, string shot 13 will be connected across transmission line lit and therefore be detonated, string shot 13 being permanently connected both to contact 35 and pole 36.

In the operation of the circuit of the invention shown in FIG. 3 with load impedance 24 being string shot 13A, free point indicator circuit 22 may be employed to pro vide a direct current voltage to detonate string shot MA by connection of free point indicator reset circuit 22 to transmission line It? via switches and 26 as before. Should conductors 27 and 2-8 at the bottom of transmission line it be blown apart, resistor 34 will provide *a current path in parallel with string shot 13A after detonation. Thus, the tool shown in FIGS. 2A and 213 may be raised in the well to position string shot 13 adjacent a tool joint immediately above the top of a set of drill collars and relay 31 energized by power supply 18 via switch 26 as before. Energization of relay 31 on application of a voltage to transmission line it by free point indicator reset circuit 22 will not detonate string shot 13 because relay 31 will not be energized in View of the fact that back bias is applied to rectifier 38 and relay winding 31 will not be permitted to draw enough current to be energized.

It can, therefore, be seen that the invention provides an unusual safety device for detonating an explosive in an oil well. The use of pole 36 with contact normally in engagement with arm 37 means that string shot 13 will 8 normally be shorted except upon energization of relay 31. This means that currents induced in transmission line it those caused by galvanic action, or salt water seepage into housing 12 shorting pole 36 to contact 38 still will not cause string shot 13 to be detonated because it is shorted through contact 35, arm 37 and pole 36.

Still further, the use of an opposite polarity lower potential direct current voltage at the output of free point indicator reset circuit 22 in combination with the use of rectifier 349 provides two additional. safety features, amplitude safety being provided by the lower amplitude output voltage of circuit 22 and polarity safety being provided by the combination of the, polarity of the output voltage of circuit 22 and the direction in which rectifier 39 is poled. The lower voltage is, of course, relative to the voltage applied by power supply 18. However, it also can be relative to that required to energize relay 31 for, of course, power supply 1&3 must provide an output voltage of a sutlicient magnitude to energize relay 31. The relative polarity is also important as far as the output voltage of power supply 18 goes in that it must be of a correct polarity to energize relay winding 31 when it is desired that string shot 13 be detonated.

t is also seen that the use of a time delay relay is helpful with the adjustment of the output voltage of power supply 13 by means of knob 19 in order to obtain proper firing current to detonate string shot 13. Still further, it is an unusual advantage of the invention that relay 3h is a temperature-compensated thermal time delay relay because of the wide variation in temperature in an oil Well and the ease with which a thermal time delay may be temperature compensated.

Summarizing, although it is a salient feature of the invention that contact 35 is normally connected to pole 36 to short string shot 13 until relay winding 31 is energized, all the other safety features of the invention are also useful including amplitude safety, polarity safety and time delay safety. It is a further outstanding feature of the invention that these may all be combined in the same circuit as shown in FIG. 3.

Although only a specific embodiment of the invention has been shown and described, other changes and modifications of the invention, of course, will suggest themselves to those skilled in the art. However, the invention is by no means to be limited to the embodiment specifically shown and described, the true scope of the invention being defined only in the appended claims.

What is claimed is:

l. A tool to be lowered into a well bore, said tool comprising: a two-conductor transmission line; a load impedance responsive to at least a first type signal applied to said transmission line connected across said line; an explosive charge responsive to at least a second type signal applied to said transmission line; and a relay having a winding, a pole, an arm connected from said pole, at first contact normally in engagement with said arm, and a second contact to engage said arm when said winding is energized, said winding being connected in said transmission line in series with one conductor thereof, said relay winding being adapted to be energized only by said second type signal, said first contact being connected to the other of said conductors and to one side of said explosive charge, said pole being connected to the other side of said explosive charge, said second contact being connected to said one of said conductors.

2. A tool to be lowered into a well bore, said tool comprising: a two-conductor transmission line; a load impedance responsive to at least a first direct-current voltage of a predetermined polarity connected across said transmission line; an explosive charge responsive to at least a second direct-current voltage having a polarity opposite that of said first direct-current voltage; a series circuit including a rectifier connected serially with switch means; and a resistor in parallel with said series circuit connected in series with one of said transmission line conductors,

said switch means normally connecting a shorting circuit across said explosive change, said rectifier being poled in a direction such that said switch means is actuable only in response to the application of said second direct current voltage to said transmission line, said switch means being actuable both to open said shorting circuit and to connect said explosive charge across said transmission line.

3. A tool to be lowered into a well here, said tool comprising: a two-conductor transmission line; a first ex plosive charge responsive to at least a first direct-current voltage of a predetermined polarity connected across said transmission line; a second explosive charge smaller than said first explosive charge responsive to at least a second direct-current voltage having a polarity opposite that of said first direct-current voltage; a series circuit including a rectifier connected serially with switch means; and a resistor in parallel with said series circuit connected in series with one of said transmission line conductors, said switch means normally connecting a shorting circuit across said explosive charge, said rectifier being poled in a direction such that said switch means is actuable only in response to the application of said second direct current voltage to said transmission line, said switch means being actuable both to open said shorting circuit and to connect said explosive charge across said transmission line.

4. A tool to be lowered into a well bore, said tool comprising: a two-conductor transmission line to be lowered into the well; means at the top of the well for selectively applying a first direct-current voltage small in amplitude and of one polarity and a second direct-current voltage of a larger amplitude and of a polarity opposite said one polarity between the conductors of said transmission line; a load impedance connected across said transmission line at a predetermined depth in the well; a first resistor connected serially with one of said transmission line conductors above said load impedance; a temperature-compensated thermal time delay relay having a winding, a pole, an arm connected from said pole, a first contact normally in engagement with said arm, and a second con tact to engage said arm when said winding is energized; a series circuit including a rectifier and said relay winding, said series circuit being connected in parallel with said first resistor,'said rectifier being poled to be conductive when said second direct-current voltage is applied between the conductors of said transmission line at the top of the well; an explosive charge, said first contact being connected to the other of said conductors and to one side of said explosive charge, said pole being connected to the other side of said explosive charge, said second contact being connected to said one of said transmission line conductors above said load impedance; and a second resistor connected in parallel with said load impedance to provide a current path for said relay in the event that the circuit of said transmission line is opened by an open in said load impedance.

5. A tool to be lowered into a well bore, said tool comprising: a two-conductor transmission line; a load impedance responsive to at least a first type signal applied to said transmisison line connected across said line; and explosive charge responsive to at least a second type signal applied to said transmission line; a shorting circuit; and switch means normally connecting said shorting circuit in parallel with said explosive charge, said switch means being actuable in response to the application of said second type signal to said transmission line and nonresponsive to the application of said first type signal to said transmission line both to open said shorting circuit and to connect said explosive charge across said transmission line, said switch means including a relay having an inductive winding and a pair of contacts normally closed to each other, energization of said inductive winding causing said contacts to open, said contacts being connected in series with said shorting circuit with one of said contacts connected to one side of said explosive charge and the other of said contacts connected to the other side of said explosive charge.

6. The invention as defined in claim 5, wherein said load impedance is an inductor coil.

7. The invention as defined in claim 5, wherein said load impedance is another explosive charge.

8. A tool to be lowered into a well bore, said tool comprising: a two-conductor transmission line; a load impedance responsive to at least a first type signal applied to said transmission line connected across said line; an explosive charge responsive to at least a second type signal applied to said transmission line; a firing circuit; switch means to connect said firing circuit to said explosive charge, said switch means actuable in response to the application of said second type signal to said transmission line and non-responsive to the application of said first type signal to said transmission line for closing said firing circuit to said explosive charge, said switch means including a time delay relay having an inductive winding and a pair of contacts noramlly opened to each other, energization of said inductive Winding causing said contacts to close at a predetermined time subsequent to said energization, said contacts being connected in series with said firing circuit.

9. A tool to be lowered into a well bore, said tool comprising: a two-conductor transmission line; a load impedance responsive to at least a first type signal applied to said transmission line connected across said line; an explosive charge responsive to at least a second type signal applied to said transmission line; a firing circuit; switch means to connect said firing circuit to said explosive charge, said switch means actuable in response to the application of said second type signal to said transmission line and non-responsive to the application of said first type signal to said transmission line for closing said firing circuit to said explosive charge, said switch means including a time-delay relay having an inductive winding and a pair of contacts normally opened to each other, energization of said inductive winding causing said contacts to close at a predetermined time subsequent to said energization, said contacts being connected in series with said firing circuit, and an adjustable voltage source connected across said transmission line at the top of the well.

10. A tool to be lowered into a Well bore, said tool comprising: a two-conductor transmission line; a load impedance responsive to at least a first type signal applied to said transmission line connected across said line; an explosive charge responsive to at least a second type signal applied to said transmission line; a firing circuit; switch means to connect said firing circuit to said explosive charge, said switch means actuable in response to the application of said second type signal to said transmission line and non-responsive to the application of said first type signal to said transmission line for closing said firing circuit to said explosive charge, said switch means including a time-delay relay having an inductive winding and a pair of contacts normally opened to each other, energization of said inductive winding causing said contacts to close at a predetermined time subsequent to said energization, said contacts being connected in series with said firing circuit, said relay being temperature-compensated to an extent such that said contacts are caused to close at substantially thesa-me energizing current thereto independent of the ambient temperature therearound.

References Qited in the file of this patent UNITED STATES PATENTS 2,331,058 Stick Oct. 5, 1943 2,732,518 Bricaud Ian. 24, 1956 2,821,136 Castel Jan. 28, 1958 2,853,012 Rotkin et a1. Sept. 23, 1958 2,871,784 Blair Feb. 3, 1959 

